The control of enzymatic activity in the catabolism of histidine and threonine and their interrelationships are the major subjects of this project. Of principal interest in histidine metabolism is the origin of the carbonyl prosthetic group of urocanase. This essential cofactor, identified as alpha-ketobutyrate, will be studied in the urocanase of Pseudomonas putida from the standpoints of its mechanism of action and its biosynthesis. With regard to the latter, the enzyme threonine dehydratase has been implicated as an essential ingredient in coenzyme formation, and possibly in overall expression of all genes concerned with histidine breakdown in this organism. Present efforts are aimed at elucidating the precise role of threonine dehydratase in these events. Threonine dehydratases from Clostridium tetanomorphum and P. putida are to be studied in order to clarify the relationships between quaternary structure and control of activity by certain allosteric effectors for each, particularly 5'-ADP and L-isoleucine. Furthermore, the absence of pyridoxal phosphate as coenzyme for the P. putida dehydratase requires that the nature of its replacement be investigated. BIBLIOGRAPHIC REFERENCES: M.S. Cohn, M.C. Lynch and A.T. Phillips. Catalytic and Thermodynamic Properties of the Urocanate Hydratase Reaction (1975) Biochim. Biophys. Acta 377, 444-453.